U.S. patent number 5,959,720 [Application Number 08/620,430] was granted by the patent office on 1999-09-28 for method for color balance determination.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to Kenneth A. Carlson, Heemin Kwon.
United States Patent |
5,959,720 |
Kwon , et al. |
September 28, 1999 |
Method for color balance determination
Abstract
A method of color balance determination for use by a color
copying apparatus utilizing a gray estimate established as a
functional relationship among at least three basic color density
values measured from regions within multiple image frames of a film
order. This functional relationship is preferably a fitted line to
a set of measured density values from which density values from
regions of high color saturation have been excluded. To
discriminate these high color saturation regions, the color
saturation is determined relative to a gray point calculated as a
weighted average of minimum density and image average density
values. Further improvement in the gray estimate is achieved by
limiting the set of measured density values to regions of high
modulance ("edge effect") within the image frames.
Inventors: |
Kwon; Heemin (Pittsford,
NY), Carlson; Kenneth A. (Rochester, NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
24485909 |
Appl.
No.: |
08/620,430 |
Filed: |
March 22, 1996 |
Current U.S.
Class: |
355/38; 355/35;
355/83 |
Current CPC
Class: |
G03B
27/73 (20130101) |
Current International
Class: |
G03B
27/73 (20060101); G03B 027/80 () |
Field of
Search: |
;355/35,38,83,68
;396/569,570,563,578 ;356/404,405 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Metjahic; Safet
Assistant Examiner: Chizmar; John
Attorney, Agent or Firm: Boos, Jr.; Francis H.
Claims
What is claimed is:
1. A method of determining the individually controllable amounts of
light in various colors to which photographic copying material is
to be exposed in a color copying operation from a length of
photographic original material including a series of discrete
image-carrying sections comprising the steps of:
individually photoelectrically measuring the density values of the
original material in at least three basic colors at a plurality of
regions from said series of discrete image-carrying sections of the
original material; and
establishing a single, multi-dimensional functional relationship
among the at least three basic colors, said relationship defining a
line representative of an exposure-level-dependent estimate of gray
for all of said discrete image-carrying sections of said entire
length of original material.
2. The method of claim 1 wherein each region is characterized
according to either one or both of color saturation and edge
determination, and said functional relationship is established from
a subset of said regions from which selected regions are excluded
on the basis of at least one of said characterizations.
3. The method of claim 2 wherein regions used in establishing said
functional relationship are edge determined regions from which
regions having color saturation exceeding a predetermined threshold
have been excluded.
4. The method of claim 1 wherein the functional relationship is a
least squares best fit line based on said density values for
regions within said series of discrete image-carrying sections.
5. The method of claim 1 wherein the functional relationship is an
approximation in the form of polynomials of at least second
order.
6. The method of claim 1 further comprising:
determining sets of minimum and average density values in each of
said colors for the length of original material;
establishing an initial gray point from a weighted average of said
minimum and average density values;
calculating color saturation relative to said initial density gray
point for regions within said series of discrete image carrying
sections; and
establishing said functional relationship from a subset of regions
which excludes regions in which said calculated color saturation
exceeds a predetermined threshold.
7. The method of claim 6 wherein frames of color saturation
exceeding a predetermined threshold value are excluded from the
establishment of said initial gray point.
8. The method of claim 6 wherein said length of original
photographic material has data recorded thereon indicative of
exposure of one or more image frames by artificial illuminant and
wherein said artificial illuminant frames are excluded from
establishment of said initial gray point.
9. The method of claim 6 wherein said length of original
photographic material has data recorded thereon indicative of
exposure of one or more image frames by artificial illuminant and
wherein said artificial illuminant frames are excluded from
establishment of said single, three dimensional functional
relationship.
10. The method of claim 1 wherein said exposure-level-dependent
estimate of gray is modified for frames having a percentage of
regions of low density exceeding a predetermined threshold
percentage.
11. The method of claim 10 wherein said method further
comprises:
determining a set of minimum density values in each of said colors
for the length of original material;
determining average density values for each of said frames in each
of said colors; and
said estimate of gray is modified by proportionate amounts of said
minimum and average density values.
Description
FIELD OF THE INVENTION
The invention relates a method and apparatus for determining color
balance for use in the copying of color original images such as in
a photographic color printer.
BACKGROUND OF THE INVENTION
Automatic color photographic printers, such as the CLAS35 printer
made by Eastman Kodak Company are known to employ color balance
algorithms for determining the amounts of exposure light in each of
a plurality of primary colors for use in exposing film images onto
copy paper. In the CLAS35 printer, the algorithms rely on film
channels with specific parameter values that are uniquely
associated with each of the different film types encountered in the
population of orders processed by the printer. This requires that a
large library of parameter values be maintained and that the
parameter values be updated as new film types are introduced.
Additionally, changes in the photometric properties of existing
film types caused, for example, by film processing errors or film
storage at high temperatures or for long periods of time can
introduce printing errors that are not compensated for by the film
type parameter values.
U.S. Pat. No. 4,279,502 discloses a method of determining color
balanced copying light amounts from photometric data derived
directly from the film without the use of film type specific
parameter values. In this method, first and second color density
difference functional correlations are established from density
values denoting the results of measurements at a plurality of
regions of the film strip which includes the original image being
copied. These correlations are then used for determining the
copying light amounts for most of the originals on the film strip.
The light amounts for originals containing illuminant error or
color dominant subjects are selected differently using empirically
determined threshold values. To be effective, this method requires
the establishment of two different, independent functional
relationships which may not capture the correct correlation among
three primary color densities in the original image. There is
therefore a need for a method of determining color balanced amounts
of copying light that is based on the establishment of a single
functional relationship among the image colors that captures the
correlation among the three primary color densities.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided a
method of determining the individually controllable amounts of
light in various colors to which photographic copying material is
to be exposed in a color copying operation from a length of
photographic original material including a series of discrete
image-carrying sections, especially from a film strip including a
series of color negatives. The method comprises the steps of
individually photoelectrically measuring the density values of the
original material in at least three basic colors at a plurality of
regions of the original material; and establishing a single,
multi-dimensional functional relationship among the at least three
basic colors representing an exposure-level-dependent estimate of
gray for use as values specific to said length of the original
material for influencing the light amount control in the color
copying operation.
These and other aspects, objects, features and advantages of the
present invention will be more clearly understood and appreciated
from a review of the following detailed description of the
preferred embodiments and appended claims, and by reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a basic block diagram of a film scanner and printer
apparatus for performing the color balance method of the
invention.
FIG. 2 is a plan view of portions of film strips showing splicing
of successive film strip orders.
FIG. 3 is a three dimensional plot of film density measurements in
three basic colors of image frames from a film strip order.
FIG. 4 is a plot of film density measurements in a transformed
color space useful in describing a method of high saturation color
discrimination for individual regions of an image frame.
FIG. 5 is a plot of film density measurements similar to FIG. 4
used in describing high saturation color discrimination for entire
image frames.
FIG. 6 is a diagram of a film image frame showing the operation of
edge detection within the image frame.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIGS. 1 and 2, reference numeral 10 denotes film
scanner apparatus and numeral 30 denotes film printer apparatus. In
scanner 10, a length of film 12 comprised of a series of separate
film strips 12a spliced together by means of adhesive connectors
13, is fed from a supply reel 14 past a splice detector 16, a notch
detector 18 and a film scanner 20 to a takeup reel 22. Splice
detector 16 serves to generate output signals that identify the
beginning and end of each separate film order which is made up of a
series of original image frames 17 on a single continuous film
strip 12a. Notch detector 18 senses notches 15 formed in the film
strip adjacent each original image and provides output signals that
are used to correlate information generated in the scanner with
specific original image frames. Scanner 20 photometrically
measures, in known manner, the density values of three primary
colors in a plurality of regions on the film strip 12a including
the original image frames 17 as well as the interframe gaps 19. The
term regions as used herein may be taken to mean individual image
pixels or groups of pixels within an image frame. The signals from
detectors 16,18 and scanner 20 are fed to a computer 24 which
operates to determine optimum exposure levels at a subsequent
printer 30. Data signals representing these optimum exposure levels
may be communicated directly to a printer computer 32 or may stored
on an intermediate storage medium 26 for later use by a printer
30.
Printer 30 operates to copy the individual images 17 from the film
12 fed from a supply reel 36 past a print gate 38 to a takeup reel
40. It will be appreciated that scanner 10 and printer 30 may be
constructed as a single system in which case the film would be fed
continuously from supply reel 14 to takeup reel 40 without
interposition of the intermediate reels 22 and 36. In such a case
it is only important that scanner 20 be located at an effective
distance from print gate 38 that allows scanning of a significant
number of original image frames 17 from a film strip 12a before the
copying operation commences. For the image copying process, printer
30 is provided with a lamp house 44 which projects the exposure
light through a series of color filters 46 and a shutter mechanism
48 which are controlled by computer 32 to control the light amounts
in each of the primary colors used to expose the original image
frames 17 from the film strip onto a strip of color copying
material 50 fed from supply reel 52 to takeup reel 54. An optical
system 56 controls the focus and magnification of the image onto
the color copying material.
As part of the operation of computer 24 in determining the optimum
exposure levels, a set of exposure dependent gray values for each
film strip 12a is derived in the form of a single, three
dimensional functional relationship among the measured three basic
color density values, such as Red, Green and Blue. This functional
relationship represents an exposure-level-dependent estimate of
gray for use as values specific to the film strip 12a for
influencing the light amount control in the color printer 30.
Conceptually, the functional relationship representing gray is
derived by creating a three dimensional scatter plot, of the type
illustrated in FIG. 3, which shows a three dimensional scatter plot
of three measured primary color densities, such as red, green and
blue, within a color space bounded by red, green and blue axes.
Although a three-dimensional functional relationship is described
herein based on the scanning of the three Red, Green and Blue
density values on the image frame, it will be appreciated that
basic color densities other than Red, Green and Blue may be scanned
to form the basis for the functional relationship. Additionally,
the invention is not limited to three color densities since more
than three color densities may be scanned. Consequently, in its
broadest form, the invention contemplates that a multi-dimensional
functional relationship is derived from at least three, and
possibly more, scanned color densities of any suitable combination
of appropriate colors.
The plotted points represent measured density values for the
corresponding colors from regions on the film strip 12a. The
functional relationship is established by plotting measured density
values 60 derived from each of the regions measured by scanner 20
in the three dimensional color space and by then fitting a line 62
through the density values 60 using any one of the many well known
line fitting methods, such as the method of least squares.
Preferably, the functional relationship is an approximation in the
form of polynomials of at least the second order. This fitted line
62 then serves as the exposure-level-dependent estimate of gray for
the film strip 12a which is then used to influence the derivation
of the optimum light exposure amounts in each color for each of the
original film image frames 17. Such derivation is accomplished by
use of known color balance algorithms such as the subject failure
suppression technique described in the journal article "Modern
Exposure Determination for Customizing Photofinishing Printer
Response", E. Goll, D. Hill and W. Severin, Journal of Applied
Photographic Engineering, Vol. 5, No. 2 (Spring 1979). While
reference is made to conceptually creating a scatter plot, this is
done for ease of visualizing the description of the invention. In
actual practice, it will be appreciated by those skilled in the art
that the gray estimate is created by inputting the density value
data into the appropriate line fitting algorithm being run by
computer 24.
In the above described method, density values 60 from all of the
measured regions in the included group of image frames within film
strip 12a were used in creating the scatter plot. It has been found
to be advantageous in some instances to exclude data from certain
regions when creating the scatter plot so as to improve the
accuracy of the gray estimate, i.e. the fitted line 62. For
example, regions with highly saturated colors such as represented
by points 64 in the plot of FIG. 3 can bias the fitted line
estimate. These high saturation regions may be caused by objects in
the photographed scene that contain highly saturated colors, an
example of which might be a person wearing bright red clothing. In
establishing the gray estimate, it is important that the estimate
mainly represent characteristics of the film strip 12a without bias
from these saturated colors in the scene. It is therefore desirable
to eliminate the density values from these saturated colors from
the scatter plot used to establish the gray estimate. In order to
eliminate the data from regions of saturated colors, it is
necessary first to establish a reference point from which
saturation values are calculated and then to establish a threshold
level relative to the reference point for use in identifying the
highly saturated colors to be eliminated.
While various techniques for elimination of highly saturated colors
may be employed, the elimination is achieved in a presently
preferred embodiment by first transforming the density values of
the three primary colors into an alternative orthogonal color space
in the manner described in commonly assigned U.S. Pat. No.
4,159,174, the disclosure of which is incorporated herein by
reference. This transformed space is illustrated in the graph of
FIG. 4, wherein the three primary color density measurements in
three dimensional space are projected onto a green/magenta,
illuminant plane which is perpendicular to a neutral axis.
Reference point 70 is established in the following manner. From the
measured density values of a plurality of regions on the film strip
12a, a set of minimum density values (R.sub.min, G.sub.min,
B.sub.min) is determined. Preferably the regions included for this
purpose are taken from both the image frames 17 and the interframe
gaps 19. The purpose is to identify an area on the film strip where
there is no exposure. Normally, this would be expected to be found
in the interframe gaps 19. However, it is known that for various
reasons there may be some exposure e.g. fogging, in the gap areas
and for this reason it is desirable to include the image frame
regions in locating the minimum color density values. Next, the
average color density values (R.sub.av, G.sub.av, B.sub.av) for all
regions within the included image frames 17 are determined. At this
point, these two sets of values are transformed into the
alternative orthogonal color space of FIG. 4 to obtain
green/magenta and illuminant values (GM.sub.min,ILL.sub.min) and
(GM.sub.av,ILL.sub.av) corresponding to minimum density values
(R.sub.min, G.sub.min, B.sub.min) and average density values
(R.sub.av, G.sub.av, B.sub.av), respectively. The weighted averages
GM.sub.o and ILL.sub.o of these values are derived from the
expressions:
where the weighting values .alpha. and .beta. are each between 0
and 1 and are empirically determined from inspection of resulting
color prints. Representative values that have been found to give
good results are approximately .alpha.=0.6 and .beta.=0.25. These
GM.sub.o and ILL.sub.o values establish the reference point 70 in
FIG. 4. It may be noted here that, while the method disclosed in
U.S. Pat. No. 4,279,502 performs a saturated color elimination
using a calculation of saturation relative solely to minimum
density (R.sub.min, G.sub.min, B.sub.min), which corresponds to
GM.sub.min and ILL.sub.min in the FIG. 4 color space, it has been
found that this does not always give optimum results. This is
believed to be because the gray point determined at minimum density
levels does not accurately reflect the gray point corresponding to
normal exposure levels. Thus, it has been found to be advantageous
to include a measure of average density of regions from within the
image frames 17 in the film strip 12a when determining the values
of GM.sub.o and ILL.sub.o.
In calculating the saturation of each of the regions, "i", in an
image frame, the measured color values may preferably be
transformed into the alternative orthogonal color space resulting
in (GM.sub.i, ILL.sub.i). A saturation value (SAT.sub.i) for a
given region in an image frame is calculated relative to the
reference point 70 (GM.sub.o,ILL.sub.o) in FIG. 4 as follows:
##EQU1##
The saturation SAT.sub.i is then compared to a predetermined
threshold value, represented by circle 72. When the saturation
SAT.sub.i is greater than the threshold value, as in the case shown
by point 74, the data from this region is excluded in creating the
scatter plot of FIG. 3. Data from regions falling within the
threshold 72, as represented by point 76, are included in the
subset used for creating the scatter plot. In terms of the FIG. 3
plot, this would result in exclusion of widely scattered points 64,
while the closely scattered points 60 are included. When the Red,
Green and Blue regions are measured in terms of density and
transformed to GM and ILL color space, the radius of threshold
circle 72 is 0.15 in a presently preferred embodiment. The
establishment of this threshold is an empirical determination and
it will be understood that other threshold values may be
established within the scope of this invention. Although the
saturation clipping technique of FIG. 4 is a simple method using a
uniform threshold shown by a circle 72 for all color directions
(hues), it will be appreciated that it is also possible to have
variable threshold values depending on the color direction similar
to the subject failure suppression boundaries technique described
in the above Goll et al journal article.
Highly saturated frames can bias the gray estimate (GM.sub.o,
ILL.sub.o). As a further refinement for estimating the values
GM.sub.o and ILL.sub.o, provision can be made to eliminate highly
saturated frames from the calculation of GM.sub.o and ILL.sub.o.
For each image frame 17, the average values GM.sub.fav and
ILL.sub.fav are calculated in a manner similar to the calculation
of GM.sub.av and ILL.sub.av as previously described, except that
only regions within the frame are included in the calculation. It
will be appreciated that GM.sub.av and ILL.sub.av are then the
averages of GM.sub.fav and ILL.sub.fav, respectively, for the
entire order. Referring now to FIG. 5, the frame saturation is
calculated relative to the previously described transformed values
GM.sub.min,ILL.sub.min, point 90 in FIG. 5. A saturation value
(SAT.sub.f) for a given image frame in an order is calculated
relative to the reference point 90 (GM.sub.min,ILL.sub.min) in FIG.
5 as follows: ##EQU2##
The saturation SAT.sub.f is then compared to a predetermined
threshold value, represented by circle 92. When the saturation
SAT.sub.f is greater than the threshold value, as in the case shown
by point 94, the values GM.sub.fav and ILL.sub.fav for this frame
are excluded in the calculation of GM.sub.av and ILL.sub.av. Data
from frames falling within the threshold 92, as represented by
point 96, are included in the calculation of GM.sub.av and
ILL.sub.av. In the special circumstance in which all the frames in
an order are excluded by this frame saturation determination, it
will be appreciated that the value of GM.sub.o and ILL.sub.o revert
to GM.sub.min and ILL.sub.min, respectively. As in the case of FIG.
4, the threshold value of circle 92 is determined empirically and
in a presently preferred embodiment, a value of 0.25 is used,
although other values may be used.
One of the effects of the just described frame elimination in
calculating GM.sub.o and ILL.sub.o is that image frames exposed
with artificial illuminant, e.g. tungsten illumination, are
eliminated. In a proposed photographic system of the type described
in commonly assigned U.S. Pat. No. 5,229,810--Cloutier et al, data
can be recorded in a camera on a magnetic layer formed on the film.
This data can include a data flag indicating that a particular
frame has been exposed by artificial illumination. This recorded
data may be used advantageously in the present invention by
detecting the data flag and causing the data values of the
corresponding frames to be excluded in the calculations of GM.sub.o
and ILL.sub.o and/or the subsequent line estimate 62 of FIG. 3.
To reduce the influence of large uniform areas of density within an
image frame that would have the effect of biasing the line estimate
62 in FIG. 3, i.e. the exposure dependent gray, preferably only
data from the high modulance regions of an image are included in
the scatter plot. High modulance regions are determined by
detecting edge regions occurring within an image frame 17. In a
preferred embodiment, a filter operating on a 3.times.3 region
matrix as shown in FIG. 6 is used. The difference between maximum
and minimum neutral density for all nine regions within the matrix
is determined and if the difference between maximum and minimum
values, i.e. the "edge value", is below a predetermined threshold
value, then the data is not included in the scatter plot. An
effective threshold value for this purpose has been found
empirically to be 0.2 neutral density in R,G,B color space or
0.2.sqroot.3 neutral density in the alternative orthogonal color
space of FIG. 4. Areas of the image with high modulation tend to
correlate well with those areas of the image that contain the
subject. Consequently, the use of edge filtering as described above
tends to include regions of the image that contain the subject and
thus provides an improved gray estimate for color balance exposure
settings according to the invention. It will be appreciated that
this aspect of edge filtering makes the edge filtering technique
also useful in determining neutral (i.e. lightness vs darkness
balance) exposure settings and in implementing the aforementioned
subject failure suppression technique.
In the case of an image frame in which a significant percentage of
the regions have density values near minimum density (R.sub.min,
G.sub.min, B.sub.min) for the order, i.e. the length of the
original material, the color correction determined by the single,
three dimensional functional relationship (line 62 in FIG. 3) may
not give optimum results for some film types. In this instance, it
is desirable to modify the color correction to accommodate these
low density frames. This may be accomplished by modifying the color
balance point, GM.sub.k, ILL.sub.k, determined from the line 62 by
proportionate amounts of GM.sub.fav, GM.sub.min and ILL.sub.fav,
ILL.sub.min. A presently preferred method for achieving this is to
initially transform Red, Green and Blue density values into neutral
values in the orthogonal color space of FIG. 4 where the neutral
axis is perpendicular to the green/magenta and illuminant axes. The
percentage "p" of low neutral density regions within the frame is
determined by counting regions with neutral densities that are
below an empirically determined neutral threshold, N.sub.t, and
dividing by the total number of regions in the frame. A weighting
factor "w" is defined as: ##EQU3## where "p.sub.o " is an
empirically determined threshold percentage of low density regions
to the total number of regions in the frame. The final balance
point GM.sub.b, ILL.sub.b for the image frame is determined by the
following equations:
where q is a smoothing parameter between GM.sub.fav, GM.sub.min and
ILL.sub.fav, ILL.sub.min, and have values between 0 and 1.
Presently preferred values for use in the foregoing process are:
N.sub.t is 0.5 above N.sub.min, where N.sub.min is the neutral
density of the minimum Red, Green and Blue densities for the length
of original material; p.sub.o is 0.6; and q is 0.5. It will be
understood that other values may be used based on empirical
tests.
The invention has been described with reference to a preferred
embodiment. However, it will be appreciated that variations and
modifications can be effected by a person of ordinary skill in the
art without departing from the scope of the invention.
______________________________________ PARTS LIST
______________________________________ 10 scanner 12 length of film
12a individual film strips 13 adhesive connector 14 supply reel 15
notches 16 splice detector 17 original image frames 18 notch
detector 19 interframe gaps 20 film scanner 22 takeup reel 24
scanner computer 26 data storage medium 30 color printer 32 printer
computer 36 supply reel 38 print gate 40 takeup reel 44 lamp house
46 color filters 48 shutter mechanism 50 color copying material 52
supply reel 54 takeup reel 56 optical system 60,64 measured density
values 62 fitted line 70 gray reference point (GM.sub.o, ILL.sub.o)
72 threshold reference circle 74,76 transformed measured density
values (GM.sub.i, ILL.sub.i) 80 3 .times. 3 region matrix 90 gray
reference point (GM.sub.min, ILL.sub.min) 92 threshold reference
circle 94,96 transformed measured density values (GM.sub.fav,
______________________________________ ILL.sub.fav)
* * * * *